In-expensive Lens for Video?

I have a friend who just bought a Rebel 3ti. She want's to use it for video and want's a low f-stop (2.8 or 3.5?) to get good depth of focus effects. She just graduated from college with a media degree and is now in the "starving artist" category so she needs to keep the price down. (Less than $300 if possible.) I don't know the Sigma or Tamron lines too well but think they might be a good option for her. Any suggestions?